1. Field of the Invention
The present invention generally relates to mobile communication terminals having functions for reporting position information. More particularly, the present invention relates to a mobile communication terminal with an SPS (Satellite Positioning System) receiver for positioning, and a positioning method of the mobile communication terminal.
2. Description of the Related Art
Conventionally, a mobile communication terminal with an SPS receiver performs positioning of mobile communication terminals using reference position information (hereinafter “RefPos”), which is one kind of assisting information. This RefPos indicates a rough position of the mobile communication terminal (for example, the position of the base station of the mobile telephone). When performing positioning, the mobile communication terminal stores the cell ID of the base station in association with RefPos, and therefore is able to reuse the positioning information from the previous RefPos as is, if the cell ID doe not change before and after move. That is, if the cell ID does not change from the previous positioning, the mobile communication terminal can use RefPos information it stores as is. By this means, the frequency the mobile communication terminal has to access the server during positioning can be reduced (e.g., see Japanese Patent Application Laid-Open No. 2005-83859).
However, the accuracy required of RefPos is generally on the order of 150 kilometers, which is substantially greater than the cell radius of the base station. Therefore, the mobile communication terminal can still use the same RefPos even when the mobile communication terminal moves to a neighboring cell (that is, to a neighboring base station). However, the SPS receiver in a conventional mobile communication terminal accesses the server and acquires RefPos when performing positioning in the stage the mobile communication terminal has moved to a neighboring cell (i.e. base station), and therefore unnecessary access to the server occurs. That is, when the mobile communication terminal moves, the mobile communication terminal has to access the server more often. In other words, when moving to a neighboring cell (i.e. base station) and performing positioning, the mobile communication terminal decides that RefPos is invalid when the RefPos really is still valid, and access the network. This results in increased load on the network, increased load on the server or increased cost of packets, leading to a problem that the operability of the mobile communication terminal deteriorates.